1. Field of the Invention
This invention relates to a plastic seal packaged semiconductor device capable of attaining high-density mounting, and more particularly to the plastic seal structure of a semiconductor device of reduced thickness, lead frame and a method of manufacturing the plastic seal structure.
2. Description of the Related Art
A semiconductor device such as an IC or LSI is packaged to protect the semiconductor substrate (chip) having semiconductor elements formed thereon from contamination source such as dusts, chemicals, gases and moisture and the mechanical damage. The package used for this purpose is required to have various characteristics including a high degree of hermetic sealing, high resistance to the high temperature condition set in the assembling step, high mechanical strength, chemical stability and excellent electrical characteristics such as insulation characteristic and high-frequency characteristic, for example, and synthetic resin or ceramics is used as the material of the package.
FIG. 1 is a cross sectional view showing plastic mold forming molds 11, 12 for forming a plastic package, a lead frame 30 disposed in a space defined by the molds, and a chip 1 mounted on the semiconductor chip mounting portion (island) 2 of the lead frame 30. FIG. 2 is a cross sectional view of a packaged SOP (Small Outline Package) type semiconductor device. Conventionally, the package for attaining the high-density mounting of a semiconductor device includes a QFP (Quad Flat Package) whose outer lead is of gull-wing type and a PLCC (Plastic Leaded Chip Carrier) whose outer lead is of J type, and further includes a TQFP (Thin Quad Flat Package) and a TSOP (Thin Small Outline Package) whose package thickness is reduced to enhance the spatial mounting density and is approx. 1.0 mm or less.
The chip 1 mounted on any type of lead frame is plastic-molded by synthetic resin 8.
A conventional semiconductor device having a package formed of synthetic resin and a method for forming the package thereof are explained with reference to FIGS. 1 and 2. As shown in FIG. 1, the chip 1 having a semiconductor element such as a memory formed thereon is fixed on the chip mounting portion 2 of the lead frame 30 by use of adhesive agent 3. The chip 2 has leads 4 for connecting the chip 1 to an external circuit. In general, bonding portions 5 are formed on respective inner leads 42 of the lead 4 and electrode pads 6 are formed on the surface of the chip 1. The bonding portions 5 are electrically connected to the electrode pads 6 via respective bonding wires 7 of Al or Au.
The chip 1, chip mounting portion 2, bonding wires 7, and inner leads 42 of the lead 4 are packaged by use of molding resin by a transfer molding method, for example. In order to form a package 8 for the packaging, a mold 10 shown in FIG. 1 is used. The mold 10 includes a drag 11 and cope 12 which respectively have cavities 13 and 14. The lead frame 30 is disposed between the drag 11 and the cope 12 such that the chip 1, chip mounting portion 2, bonding wires 7 and the like may be arranged in a concave portion defined by the cavities of the drag and cope, and then mold resin 81 supplied from the exterior is injected into the concave portion via a runner 16 and a gate 17 which are formed in the drag 11 so as to form the package 8 shown in FIG. 2. After this, tie bars 19, 23 of the lead frame 30 (refer to FIG. 3) for preventing the outflow of resin at the time of formation of the package, chip mounting portion supporting member 18 and other unnecessary portions are removed, and then outer lead portions 41 of the lead 4 which extend outwardly from the package 8 are plated and bent into a preset lead form so as to complete the semiconductor device of FIG. 2. The front end portions of the outer lead portions 41 of the lead 4 of the semiconductor device are electrically connected to corresponding wiring layers of the circuit board by soldering, for example. Thus, the semiconductor device is mounted on the circuit board 9.
In the above semiconductor device, since the leads exposed from the package are not fixed in the package having a gull wing type lead form, the leads are easily bent, providing a large obstacle to the attainment of smaller pitch size. In the J type PLCC, the degree of bending of the leads can be suppressed by forming a pocket in the rear surface of the package and bending the lead terminals into the pocket, but the thickness of the package must be made large enough to form the pocket therein and it is difficult to make the wall thickness of the package equal to or less than 1 mm. It is generally known that the leads extending outwardly from the side surface of the package are weak to mechanical shock. Further, the leads extending from the package of thin type are of gull wing type, and if a semiconductor device having a package with a package thickness T of 1 mm is mounted on the circuit board 9 as shown in FIG. 2, a space is formed between the bottom surface of the package and the surface of the circuit board 9 and the distance therebetween is as long as 100 to 200 .mu.m and is not constant. When a memory card is formed, a plurality of such semiconductor devices are stacked on the circuit board, and therefore, it is extremely important to make the space small and suppress a variation in the size of the space to minimum.
In order to enhance the mechanical strength of the leads exposed from the package, that is, the outer leads, it is proposed to make the thickness of the outer lead larger than that of the lead portions (inner leads) buried in the package (refer to Jpn. Pat. Appln. KOKAI Publication No. 4-53252). With this technique, the outer lead is formed with a swelling portion having an increased thickness and is formed to extend outwardly with the swelling portion fixed on the side wall and bottom wall of the package by use of resin. However, even if the mechanical strength is enhanced as described above, the outer leads are formed to extend out from the package and cannot sufficiently cope with the high-density mounting which will be further enhanced. Further, in the package forming step, a normal mold is used, but the mold must be formed with openings for permitting passage of the swelling portions of the outer leads. Since the plastic molding is effected with the inner leads disposed between the cope and drag of the mold, a space corresponding to the thickness of the leads is provided between the cope and drag and the resin may flow out from the space in some cases. In order to prevent the outflow, it is necessary to form openings in one of the cope and drag of the mold. In addition, the mold itself is not designed to make the thickness of the package thin.